Cobalt chromite nanoparticles on multi-walled carbon nanotubes for electrochemical sensing of nitrite in water samples

The regular discharge of high concentrations of nitrite into water resources has been caused by increased anthropogenic activities and the overuse of nitrogen-based fertilizers, posing a threat to the environment and public health. In this work, cobalt chromite nanoparticles (Co(Cr₂O₄)) were synthesized by co-precipitation and calcined at three different temperatures (300 °C, 400 °C, and 700 °C) to optimise the nanoparticles for better characteristics. The acceptable Co(Cr₂O₄)@400 °C nanoparticle was composited with multi-walled carbon nanotubes (MWCNTs) at a weight ratio of 1:1, 1:2, and 1:3 for the electrochemical sensing of nitrite. Extensive physicochemical studies confirmed spinel structure, uniform incorporation of MWCNTs, and thermal stability of the prepared materials. Due to strong interfacial interactions, Co(Cr₂O₄)@400 °C/MWCNT-1:3 presented the best morphology and stability among the composites. Co(Cr₂O₄)@400 °C/MWCNT catalyst exhibited an excellent sensitivity of 270 μA/μM cm² towards nitrite and demonstrated better stability (RSD = 0.96%) and reproducibility (RSD = 1.24%). The ability for the selective determination of nitrite in the presence of various interferents (RSD = 1.13%) and the limit of detection (LOD) of 0.1 μM was examined. The sensing performance of the Co(Cr₂O₄)@400 °C/MWCNT electrode was studied by determining the nitrite in the tap water, and the recovery rates were observed as 96.8%, 104.8%, 104.6%, and 103.2% for 0.08 mM, 0.12 mM, 0.16 mM, and 0.20 mM concentrations of the nitrite.